Water Quality Performance And Greenhouse Gas Flux Dynamics From Compost-Amended Bioretention Systems & Potential Trade-Offs Between Phytoremediation And Water Quality Stemming From Compost Amendments

Stormwater runoff from existing impervious surfaces needs to be managed to protect downstream waterbodies from hydrologic and water quality impacts associated with development. As urban expansion continues at a rapid pace, increasing impervious cover, and climate change yields more frequent extreme precipitation events, increasing the need for improved stormwater management. Although green infrastructure such as bioretention has been implemented in urban areas for stormwater quality improvements and volume reductions, these systems are seldom monitored to validate their performance. Herein, we evaluate flow attenuation, stormwater quality performance, and nutrient cycling from eight roadside bioretention cells in their third and fourth years of implementation in Burlington, Vermont. Bioretention cells received varying treatments: (1) vegetation with high-diversity (7 species) and low-diversity plant mixes (2 species); (2) proprietary SorbtiveMediaTM (SM) containing iron and aluminum oxide granules to enhance sorption capacity for phosphorus; and (3) enhanced rainfall and runoff (RR) to certain cells (including one with SM treatment) at three levels (15%, 20%, 60% more than their control counterparts), mimicking anticipated precipitation increases from climate change. Bioretention water quality parameters monitored include total suspended solids (TSS), nitrate/nitrite-nitrogen (NOx), ortho-phosphorus (Ortho-P), total nitrogen (TN) and total phosphorus (TP), which were compared among bioretention cells’ inflows and outflows across 121 storms. Simultaneous measurements of flow rates and volumes allowed for evaluation of the cells’ hydraulic performances and estimation of pollutant load and event mean concentration (EMC) removal. We also monitored soil CO2 and N2O fluxes, as they represent a potential nutrient loss pathway from the bioretention cells. We determined C and N stocks in the soil media and vegetation, which are critical design elements of any bioretention, to determine the overall C and N balances in these systems. Significant average reductions in effluent stormwater volumes and peak flows were reported, with 31% of the storms events completely captured. Influent TSS loads and EMCs were well retained by all cells irrespective of treatments, storm characteristics, or seasonality. Nutrient removal was treatment-dependent, where the SM treatments consistently removed P loads and EMCs, and sometimes N as well. The vegetation and RR treatments mostly exported nutrients to the effluent. We attribute observed nutrient exports to the presence of excess compost in the soil filter media. Rainfall depth and peak inflow rate undermined bioretention performance, likely by increasing pollutant mobilization through the filter media. While the bioretention cells were a source of CO2, they varied between being a sink and source of N2O. CO2 fluxes were orders of magnitude higher than N2O fluxes. However, soil C and N, and plant C and N in biomass was seen to largely offset respiratory CO2-C and biochemical N2O-N losses from bioretention soil. The use of compost in bioretention soil media should be reduced or eliminated. If necessary, compost with low P content and high C: N ratio should be considered to minimize nutrients losses via leaching or gas fluxes. In order to understand trade-offs stemming from compost amendments, we conducted a laboratory pot study utilizing switchgrass and various organic soil amendments (e.g., different compost types and coir fiber) to a sandy loam soil contaminated with heavy metals and studied potential nutrient leaching and pollutant uptake. Addition of organic amendments significantly reduced metal bioavailability, and improved switchgrass growth and metal uptake potential. While no differences in soil or plant metal uptake were observed among the amendments, significant differences in nutrient leaching were observed

Nutrient Leaching from Compost: Implications for Bioretention and Other Green Stormwater Infrastructure

This work is made available under the terms of the Creative Commons Attribution 4.0 International license,. Compost is often used as a soil amendment in gardens, agricultural fields, and other landscaped systems to alter soil biophysical characteristics and increase availability of valuable nutrients including nitrogen (N), phosphorus (P), and carbon (C). However, leaching of soluble nutrients from compost is of concern, particularly in wet settings, such as within green stormwater infrastructure, riparian areas, and floodplains. This research highlights the importance of saturation as an influencing factor on the nutrient leaching potential of different composts and compost-amended bioretention soils. Nutrient leaching potential was evaluated for five different compost types and two compost-amended bioretention soil mixes under increasing saturation durations, measured at 10 min, 1 day, 5 days, and 10 days of saturation. Results indicated significant increases in NH4+ concentrations in leachate for all composts and bioretention media from 10 min to 10 days. Over the same time period results showed decreases in NO3- concentrations in the leachate from all five composts, but an increase in NO3- concentration for one compost-amended bioretention media and no significant change in the other bioretention media. In response to increased saturation durations, PO43- concentrations in the leachate were found to significantly increase at each stage, from 10 min, to 1 day, to 5 days, to 10 days; overall there were higher PO43- concentrations in the leachate from the five composts than in the leachate from the two bioretention mixes

Soil media CO\u3csub\u3e2\u3c/sub\u3e and N\u3csub\u3e2\u3c/sub\u3eO fluxes dynamics from sand-based roadside bioretention systems

Green stormwater infrastructure such as bioretention is commonly implemented in urban areas for stormwater quality improvements. Although bioretention systems\u27 soil media and vegetation have the potential to increase carbon (C) and nitrogen (N) storage for climate change mitigation, this storage potential has not been rigorously studied, and any analysis of it must consider the question of whether bioretention emits greenhouse gases to the atmosphere. We monitored eight roadside bioretention cells for CO2-C and N2O-N fluxes during two growing seasons (May through October) in Vermont, USA. C and N stocks in the soil media layers, microbes, and aboveground vegetation were also quantified to determine the overall C and N balance. Our bioretention cells contained three different treatments: plant species mix (high diversity versus low diversity), soil media (presence or absence of P-sorbent filter layer), and hydrologic (enhanced rainfall and runoff in some cells). CO2-C and N2O-N fluxes from all cells averaged 194 mg m-2 h-1 (range: 37 to 374 mg m-2 h-1) and 10 μg m-2 h-1 (range: -1100 to 330 μg m-2 h-1), respectively. There were no treatment-induced changes on gas fluxes. CO2-C fluxes were highly significantly correlated with soil temperature (R2 = 0.68, p \u3c 0.0001), while N2O-N fluxes were weakly correlated with temperature (R2 = 0.017, p = 0.04). Bioretention soil media contained the largest pool of total C and N (17122 g and 1236 g, respectively) when compared with vegetation and microbial pools. Microbial biomass C made up 14% (1936 g) of the total soil C in the upper 30 cm media layer. The total C and N sequestered by bioretention plants were 13,020 g and 320 g, respectively. After accounting for C and N losses via gas fluxes, the bioretention appeared to be a net sink for those nutrients. We also compared our bioretention gas fluxes to those from a variety of natural (i.e., grasslands and forests) and artificial (i.e., fertilized and irrigated or engineered) land-use types. We found bioretention fluxes to be in the mid-range among these land-use types, mostly likely due to organic matter (OM) influences on decomposition being similar to processes in natural systems

Effects of different soil media, vegetation, and hydrologic treatments on nutrient and sediment removal in roadside bioretention systems

Water quality performance of eight roadside bioretention cells in their third and fourth years of implementation were evaluated in Burlington, Vermont. Bioretention cells received varying treatments: (1) vegetation with high-diversity (7 species) and low-diversity plant mix (2 species); (2) proprietary SorbtiveMedia™ (SM) containing iron and aluminum oxide granules to enhance sorption capacity for phosphorus; and (3) enhanced rainfall and runoff (RR) to certain cells (including one with SM treatment) at three levels (15%, 20%, 60% more than their control counterparts), mimicking anticipated precipitation increases associated with climate change. A total of 121 storms across all cells were evaluated in 2015 and 2016 for total suspended solids (TSS), nitrate/nitrite-nitrogen (NOx), ortho-phosphorus (Ortho-P), total nitrogen (TN) and total phosphorus (TP). Heavy metals were also measured for a few storms, but in 2014 and 2015 only. Simultaneous measurements of flow rates and volumes allowed for evaluation of the cells’ hydraulic performances and estimation of pollutant load removal efficiencies and EMC reductions. Significant average reductions in effluent stormwater volumes (75%; range: 48–96%) and peak flows (91%; range: 86–96%) was reported, with 31% of the storms events (all less than 25.4 mm (1 in.), and one 39.4 mm (1.55 in.)) depth completely captured by bioretention cells. Influent TSS concentrations and event mean concentrations (EMCs) was mostly significantly reduced, and TSS loads were well retained by all bioretention cells (94%; range: 89–99%) irrespective of treatments, storm characteristics or seasonality. In contrast, nutrient removal was treatment-dependent, where the SM treatments consistently removed P concentrations, loads and EMCs, and sometimes N as well. The vegetation and RR treatments mostly exported nutrients to the effluent for those three metrics with varying significance. We attribute observed nutrient exports to the presence of excess compost in the soil media. Rainfall depth and peak inflow rate had consistently negative effects on all nutrient removal efficiencies from the bioretention cells likely by increasing pollutant mobilization. Seasonality followed by soil media presence, and antecedent dry period were other predictors significantly influencing removal efficiencies for some nutrient types. Results from the analysis will be useful to make bioretention designers aware of the hydrologic and other design factors that will be the most critical to the performance of the bioretention systems in response to interactive effects of climate change

Assessing the Potential of Urban Ecology Research to Inform Municipal Sustainability Practices

Cities are increasingly making decisions related to sustainability, and information from the field of urban ecology may be useful in informing these decisions. However, the potential utility of this information may not translate into it actually being used. We surveyed municipal sustainability staff through the Minnesota GreenStep Cities program documenting their information needs and information sources, and used these results to identify the frequency with which urban ecologists are publishing studies of potential relevance to practitioners. We also quantified funded awards from the U.S. National Science Foundation in urban ecology that explicitly describe active partnerships with city policy makers. Our results show that urban ecologists are increasingly generating information of potential relevance to city sustainability efforts, with rapid increases in the number of articles published and grants funded on areas identified as key information needs. Our results also suggest that the transmission of information from academic urban ecologists to practitioners occurs mostly through indirect pathways, as municipal sustainability staff reported relying heavily on general web searches and government agency websites to find information. We found evidence of an increasing frequency of active collaborations between urban ecologists and policy makers from NSF grant abstracts. Our findings are consistent with previous findings that traditional models of passive communication to practitioners through academic journals results in a low efficiency of use of this knowledge, but that the potential for urban ecologists to help inform municipal sustainability initiatives through active collaborations with practitioners is great

Quantifying nutrient recovery efficiency and loss from compost-based urban agriculture.

The use of compost in urban agriculture offers an opportunity to increase nutrient recycling in urban ecosystems, but recent studies have shown that compost application often results in phosphorus (P) being applied far in excess of crop nutrient demand, creating the potential for P loss through leachate and runoff. Management goals such as maximizing crop yields or maximizing the mass of nutrients recycled from compost may inadvertently result in P loss, creating a potential ecosystem disservice. Here, we report the results from the first two years of an experimental study in which four different crops grown in raised-bed garden plots with high background P and organic matter received one of two types of compost (municipal compost made from urban organics waste, or manure-based compost) at two different levels (applied based on crop N or P demand), while additional treatments received synthetic N and P fertilizer or no soil amendments. Because of the low N:P ratio of compost relative to crop nutrient uptake, compost application based on crop N demand resulted in overapplication of P. Crop yield did not differ among treatments receiving compost inputs, and the mass of P recovered in crops relative to P inputs decreased for treatments with higher compost application rates. Treatments receiving compost targeted to crop N demand had P leachate rates approximately twice as high as other treatments. These results highlight tradeoffs inherent in recycling nutrients through UA, but they also show that targeted compost application rates have the capacity to maintain crop yields while minimizing nutrient loss. UA has the potential to help close the urban nutrient loop, but if UA is to be scaled up in order to maximize potential social, economic, and environmental benefits, it is especially important to carefully manage nutrients to avoid ecosystem disservices from nutrient pollution

Phytoremediation of Heavy Metal-Contaminated Soil by Switchgrass: A Comparative Study Utilizing Different Composts and Coir Fiber on Pollution Remediation, Plant Productivity, and Nutrient Leaching

We investigated the effects of organic amendments (thermophilic compost, vermicompost, and coconut coir) on the bioavailability of trace heavy metals of Zn, Cd, Pb, Co, and Ni from heavy metal-spiked soils under laboratory conditions. To test switchgrass (Panicum virgatum) as a potential crop for phytoremediation of heavy metal from soil, we investigated whether the addition of organic amendments promoted switchgrass growth, and consequently, uptake of metals. Compost is a valuable soil amendment that supplies nutrients for plant establishment and growth, which is beneficial for phytoremediation. However, excess application of compost can result in nutrient leaching, which has adverse effects on water quality. We tested the nutrient leaching potential of the different organic amendments to identify trade-offs between phytoremediation and water quality. Results showed that the amendments decreased the amount of bioavailable metals in the soils. Organic amendments increased soil pH, electrical conductivity (EC), and soil nutrient status. Switchgrass shoot and root biomass was significantly greater in the amended soils compared to the non-amended control. Amended treatments showed detectable levels of heavy metal uptake in switchgrass shoots, while the control treatment did not produce enough switchgrass biomass to measure uptake. Switchgrass uptake of certain heavy metals, and concentrations of some leachate nutrients significantly differed among the amended treatments. By improving soil properties and plant productivity and reducing heavy metal solubility that can otherwise hamper plant survival, organic amendments can greatly enhance phytoremediation in heavy metal-contaminated soils

Measuring the Fate of Compost-Derived Phosphorus in Native Soil below Urban Gardens

The heavy reliance on compost inputs in urban gardening provides opportunities to recycle nutrients from the urban waste stream, but also creates potential for buildup and loss of soil phosphorus (P). We previously documented P in leachate from raised-bed garden plots in which compost had been applied, but the fate of this P is not known. Here, we measured P concentrations in soils below four or six-year-old urban garden plots that were established for research. We hypothesize that the soil P concentration and depth of P penetration will increase over time after gardens are established. Soil cores were collected in five garden plots of each age and quantified for inorganic weakly exchangeable P. Inorganic weakly exchangeable P was significantly elevated in native soil below garden plots (>35 cm deep) relative to reference soil profiles, and excess P decreased with increasing depth, although differences between garden plots of different ages were not significant. Our analysis shows that excess P from compost accumulates in native soil below urban garden plots. While urban agriculture has the potential to recycle P in urban ecosystems, over-application of compost has the potential to contribute to soil and water pollution

Urban Heat Island Mitigation Due to Enhanced Evapotranspiration in an Urban Garden in Saint Paul, Minnesota, USA

As a result of extensive urban development coupled with warming temperatures, urban heat islands (UHI) have become an important factor affecting energy consumption and human health in cities. Prior research has shown that evapotranspiration (ET) from urban vegetation can have a significant cooling effect, but there are relatively few direct measurements from urban vegetable gardens. We compared hourly temperature measurements during two summers (2017 and 2018) in a 750 m 2 research garden at the University of St. Thomas (Saint Paul, Minnesota, USA) to hourly temperatures at the nearby Minneapolis-Saint Paul (MSP) International Airport, located 6 km to the south. We also quantified seasonal ET (June–October) in 132 garden plots and five reference turfgrass plots during the summers of 2017 and 2018. For both years, an increase in temperature of 1.00°C at the MSP airport resulted in an average increase of 0.55°C in the research garden. At temperatures greater than 22°C, the garden was cooler on average compared to MSP airport. ET in the garden plots was significantly higher than in the grass reference plots both years, with means of 46 cm for garden plots compared to 19 cm for grass plots in 2017, and 51 cm for garden plots compared to 33 cm for grass plots in 2018. These results are consistent with other research showing potentially large benefits of cooling through ET from urban gardens that are primarily aimed at crop production

Soil CO2 Efflux and Root Productivity in a Switchgrass and Loblolly Pine Intercropping System

Switchgrass intercropped with loblolly pine plantations can provide valuable feedstock for bioenergy production while providing ancillary benefits like controlling competing vegetation and enhancing soil C. Better understanding of the impact of intercropping on pine and switchgrass productivity is required for evaluating the long-term sustainability of this agroforestry system, along with the impacts on soil C dynamics (soil CO2 efflux; RS). RS is the result of root respiration (RA) and heterotrophic respiration (RH), which are used to estimate net C ecosystem exchange. We measured RS in intercropped and monoculture stands of loblolly pine (Pinus taeda L.) and switchgrass (Panicum virgatum L.). The root exclusion core technique was used to estimate RA and RH. The results showed pure switchgrass had significantly higher RS rates (July, August and September), root biomass and length relative to intercropped switchgrass, while there were no significant changes in RS and roots between intercropped and monoculture loblolly pine stands. A significant decrease in switchgrass root productivity in the intercropped stands versus monoculture stands could account for differences in the observed RS. The proportions of RS attributed to RA in the intercropped stand were 31% and 22% in the summer and fall respectively, indicating that the majority of the RS was heterotrophic-driven. Ancillary benefits provided by planting switchgrass between unutilized pine rows can be considered unless the goal is to increase switchgrass production